Prevent performing both transaction and payment checkout process at the same time

[IKarbowiak]

Adjust the checkout process to prevent performing both transaction and payment checkout process at the same time.

• Add validation in transactionInitialize and transactionProcess mutations - raise an error when the checkout is currently during the complete process.
• Adjust the checkout flow selection:
  • choose the transaction flow if:
    • the checkout is fully authorized (checkout.authorize_status == CheckoutAuthorizeStatus.FULL)
    • OR the checkout is not fully authorized, has transactions and there is no active payment object
    • OR checkout amount is 0 and channel has TRANSACTION_FLOW chosen as marked_as_paid_strategy
  • choose the payment flow
    • there are no transactions
    • OR checkout is not fully authorized and has active payment

Impact

• [ ] New migrations
• [ ] New/Updated API fields or mutations
• [ ] Deprecated API fields or mutations
• [ ] Removed API types, fields, or mutations

Docs

• [ ] Link to documentation:

Pull Request Checklist

• [ ] Privileged queries and mutations are either absent or guarded by proper permission checks
• [ ] Database queries are optimized and the number of queries is constant
• [ ] Database migrations are either absent or optimized for zero downtime
• [ ] The changes are covered by test cases
• [ ] All new fields/inputs/mutations have proper labels added (ADDED_IN_X, PREVIEW_FEATURE, etc.)
• [ ] All migrations have proper dependencies
• [ ] All indexes are added concurrently in migrations
• [ ] All RunSql and RunPython migrations have revert option defined